Check out Jet Performance. They custom build Holley carburetors of just about any size for individual applications. I had them make one of their stage 2 750's for my 428. I've only had it running on a starter stand at this point, so I can't tell yet how it will act on the street. Seems to run great on the stand. May not be what you're looking for, but you should check it out. My stage 2 750 with vacum secondaries and electric choke cost about $500.
Go to
www.jetchip.com then click on fuel systems then click on carburetors. A page will come up with all the different models they work with.